Abstract: A laser beam is expanded by means of a beam expander device, the expanded beam striking the surface of a mirror which has an aperture formed therein. A portion of the beam passes through the aperture and is focused so that it strikes an optical detector. The major portion of the beam is reflected from the mirror surface to a scanner mirror which provides a transmitter beam to scan targets to be detected. A beam received back from the targets also strikes the scanner mirror and is reflected by this scanner mirror to a corner reflector, the beam being displaced a predetermined amount by the corner reflector and reflected therefrom in a direction precisely opposite to that of the beam incident on the corner reflector. This beam, after being reflected by the corner reflector, strikes a reflective surface of th apertured mirror directly opposite to that on which the laser beam is incident.